Optical communication systems are known which carry an optical channel of a single wavelength over one or more optical fibers. To convey information from plural sources, time-division multiplexing (TDM) is frequently employed. In TDM, a particular time slot is assigned to each signal source, and the complete signal is constructed from portions associated with each time slot. While this is a useful technique for carrying plural information sources on a single optical channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single optical fiber with each channel being assigned a particular wavelength. Since each optical channel itself can be time division multiplexed, the overall information carrying capacity of the fiber optic network can be increased substantially.
Optical channels in a WDM system are frequently transmitted over silica based optical fibers, which typically have relatively low loss at wavelengths within a range of 1520 to 1580 nm. WDM optical signal channels at wavelengths within this low loss “window” can be transmitted over distances of approximately 50 km without significant attenuation. For distances beyond 50 km, however, optical amplifiers are required to compensate for optical fiber loss.
Optical amplifiers have been developed which include a gain medium doped with a rare earth element, such as erbium, praseodymium, neodymium, and tellurium. The most commonly used rare earth element is erbium because it produces the greatest gain within the wavelength range of 1520 to 1580 nm. In particular, this wavelength range covers the wavelength range referred to as the conventional or C-band range, which approximately extends from 1525 to 1565 nm. The erbium doped medium is “pumped” with light at a selected wavelength, e.g., 980 nm, to provide amplification or gain at wavelengths within this low loss window of the optical fiber.
However, a shortcoming of erbium doped amplifiers is that their efficiency greatly decreases outside of the C-band range. For example, in order to further increase the bandwidth of WDM systems, optical signal transmissions in the 1565 to 1610 nm wavelength range, the so-called Long (L-band) wavelength range, and optical signal transmission in the 1450 to 1500 nm wavelength range, the so-called short (S-band) wavelength range are being combined with optical transmissions in the C-band range. However, because the L-band and S-band are far from the erbium ion absorption band, the power conversion efficiency of an erbium doped amplifier is too low to get a high gain. Therefore, in broadband WDM transmission systems, optical transmission systems that transmit optical signals that span the S-band, C-band, and L-band, erbium doped optical amplifiers prove to be inefficient.
Several methods have been previously proposed to improve the L-band gain of erbium doped amplifiers, such as applying unwanted C-band amplified spontaneous emission (ASE), using a double pass configuration, and a reflection-type erbium doped fiber amplifier with fiber grating. However, these methods have failed to produce an erbium doped amplifier capable of broadband amplification, while attaining sufficient gains and noise figure. Accordingly, there is a need for an erbium doped amplifier capable of providing sufficient gain and noise figure across the broadband wavelength range.